1. Field of the Invention
The field of the present invention is that of the transmission of electromagnetic signals, especially microwave signals, in particular the switching of such signals, and the present invention consists in a single-pole double-throw switch with no single failure point.
The present invention finds one application in signal processing systems having a structure in which functional modules are organized to obtain two for one redundancy.
2. Description of the Prior Art
Some types of equipment, especially equipment intended to be installed on board satellites, include functional modules that are duplicated so that a failure affecting the operation of one module can be remedied by starting up an identical or similar module that duplicates it in a parallel branch.
This kind of redundancy, known as two for one redundancy, is conventionally applied to diverse modules included in onboard equipment, for example amplifier modules for operating on signals that are transmitted via waveguides.
In cases of two for one redundancy of this kind, it is currently conventional to use a mechanical single-pole double-throw (SPDT) switch, for example a T switch, which remains in a previously assigned state in the absence of any command. It follows from this property that an appropriate command is necessary to change the state of the switch.
FIG. 1 shows an example of a mechanical switch of this kind which, in configuration 1 (FIG. 1A) allows the signal to propagate from the port 1 to the port 3 (port 2 being barred to the signal), after which, following an appropriate command or actuation (configuration 2—FIG. 1B), the switch allows the signal to propagate from port 1 to port 2 (port 3 being barred to the signal).
Because the mechanical switch remains in a given functional state in the absence of a command, it does not constitute a single failure point.
However, these mechanical switches are bulky and relatively heavy, and their use therefore represents a penalty in some applications, in particular in space applications.
To overcome these drawbacks and limitations, it has previously been proposed to replace mechanical switches with switching devices using electronic components.
However, in the absence of a command (energization current or voltage), because of the symmetry of the channels of the switch, an SPDT switch using solid state electronic components (diodes, transistors, etc.) is in an indeterminate and non-functional configuration.
To explain the problem arising from this situation more clearly, the operation of prior art electronic switches of this kind is explained below with reference to FIGS. 2 and 3 of the appended drawings, both in the case of a series configuration (FIG. 2) and in the case of a parallel configuration (FIG. 3).
In FIG. 2, the electrical distance between the points A and B, on the one hand, and the points A and C, on the other hand, is equal to an integer N multiple of half the wavelength λ of the signal. The devices for switching from one channel to the other are disposed in series on each of the channels and the commands applied to the device of each channel are complementary. In theory, the switch operates as follows:                Case 1: a command sent to the device on channel 1-2 (1 to 2) causes the latter to behave like a short circuit. A complementary command sent to the device on channel 1-3 causes the latter then to behave as an open circuit. As seen from the branch at the point A, the channel 1-2 is adapted while the channel 1-3 is open circuit. Channel 1-2 is therefore open and channel 1-3 closed.        Case 2: this is complementary to case 1. Channel 1-3 is open and channel 1-2 is closed.        
In FIG. 3, the electrical distance between the points A and B, on the one hand, and the points A and C, on the other hand, is equal to an odd integer Nodd multiple of one quarter of the wavelength λ of the signal. The devices for switching from one channel to the other are disposed in parallel on each channel. The commands applied to the devices of each channel are complementary. In theory, this switch operates as follows:                Case 1: a command sent to the device on channel 1-2 causes the latter to behave as a short circuit. The impedance as seen from the branch at the point A is an open circuit. A complementary command sent to the device on channel 1-3 causes the latter to behave as an open circuit. Being disposed in parallel, the latter is therefore transparent. As seen from the branch at the point A, channel 1-3 is adapted while channel 1-2 is open circuit. Channel 1-3 is therefore open and channel 1-2 is closed.        Case 2: this is complementary to case 1. Channel 1-2 is open and channel 1-3 is closed.        
Note that, because of the symmetry of the switch, the commands applied to each channel must necessarily be complementary if the devices are identical (transistors of the same kind, diodes, etc.). Accordingly, if the commands do not reach one of the devices, and even more so if they fail to reach both devices, the channels behave neither as short circuits nor as open circuits. The signal then propagates simultaneously on both channels and is subject to at least the splitting losses, i.e. losses of 3 dB (there is half of the signal on each channel). If the switch is upstream of a receive head, these losses are unacceptable from a system point of view. The switch then constitutes a single failure point.
Switches in which the problem referred to above is present are described in U.S. Pat. Nos. 4,316,159, 4,779,065 and 5,696,470, for example, in relation to single, double or matrix switching arrangements, associated or not with two for one redundant systems.
To eliminate the switches or the like active switching devices as such, it has also been proposed to provide a waveguide in the form of a hollow tube adapted to transmit electromagnetic signals and in the wall of which are mounted two coupling probes (electromagnetic field sampling devices) that are connected to two duplicated functional branches of a two for one redundant system. A solution of this kind is disclosed in the document WO 01/82405, for example.
One or the other of the two branches of the system is selected by activating the first processing module of one of the two branches and adapting the impedance of that branch in a corresponding fashion to sample and transmit electromagnetic energy from signals propagating in the waveguide, the other branch having a reflective impedance (no transmission).
However, this solution necessitates the provision of a structure forming a waveguide and provided with two sampling probes (i.e. a precise mechanical assembly) and generally with additional impedance matching modules at the output of each probe, resulting in an overall structure that is bulky, complex and costly.
A particular object of the present invention is to overcome the drawbacks previously cited.